Antihistamines

Introduction

Antihistamines interfere with the binding of histamine to its receptors. There are three types of antihistamine receptors: H1, H2 and H3. H1 receptors are important in allergic reactions and their blockade by antihistamines reduces symptoms such as itching, rash and vasodilation. These are absorbed rapidly from the gastrointestinal tract and metabolised in the liver.

H1 receptor antagonists

The classical antihistamines such as chlorpheniramine are effective H1 blockers but sedation is prominent. There is little evidence to suggest that one antihistamine is better in effectiveness than others, though individual response may vary widely. The duration of action and side-effect profile may determine

Table 5.4 Commonly used sedative and non-sedative antihistamines

Sedative antihistamines

Non-sedative antihistamines

Chlorpheniramine

Clemastine

Hydroxyzine

Promethazine

Cyproheptadine

Azatadine

Acrivastine Terfenadine Astemizole

Fexofenadine

Loratadine Cetirizine

Brompheniramine Trimeprazine the choice. The second-generation antihistamines are at least equally effective and are much less sedating (Table 5.4).

Indications

In the treatment of food allergy, antihistamines are given primarily to relieve symptoms such as itching and urticaria due to inadvertent exposure. Oral symptoms, such as itching in the mouth and throat and swelling, may also respond but there is little effect on gastrointestinal symptoms such as vomiting and diarrhoea. For mild symptoms, oral antihistamine may be effective and may be continued until symptoms disappear. For moderate to severe allergic reactions, antihistamine should be given through the parentral route for rapid systemic availability. Occasionally antihistamines are used regularly for chronic food allergic symptoms where causative food(s) have not been identified.

Side effects

Drowsiness and antimuscarinic effects such as urinary retention, dry mouth and blurred vision are major disadvantages with older antihistamines. The so-called non-sedating antihistamines can also cause drowsiness in some patients. Arrhythmias may occur in high doses, particularly with terfenadine and astemizole.

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